A technique of forming a thin film transistor (TFT) by using a thin semiconductor film that is formed over a substrate having an insulating surface has attracted attention. A thin film transistor is used for a display device typified by a liquid crystal television. Besides a silicon-based semiconductor material which is known as a material for a semiconductor thin film applicable to a thin film transistor, an oxide semiconductor has attracted attention.
As a material for the oxide semiconductor, zinc oxide and a material containing zinc oxide as its component are known. In addition, a thin film transistor including an amorphous oxide (oxide semiconductor) electron carrier concentration of which is lower than 1018/cm3 is disclosed (see Patent Documents 1 to 3).